Increasing wind penetration National wind characteristics and power generation

Studies of wind speeds in Great Britain show that there are significant periods in an average year when demand is high and wind output over the whole country is low. In particular, a typical year would have over 1600 hours when wind-generated output would be less than 10 per cent of maximum rated installed wind-generation capacity, including 450 hours when demand is between 70 to 100 per cent of peak demand (OXERA, 2003). Although the risk of system failure is greatest when demand is at its absolute peak, the risk is still significant for demands within a few percentage of the peak, say within 2GW to 4GW of peak in the system of the National Grid. Previous studies noted that thermal plant output may have a standard deviation of between 1GW and 2GW around the peak availability (Grubb, 1988).

Generally, in Great Britain stronger winds occur in winter; thus, during the winter season the average winter wind power available exceeds the mean annual level. Previous studies (Grubb, 1988) have shown, however, that the correlation with the peak 1 per cent of demand is negative, suggesting a tendency for the very highest levels of demand to be associated with less wind energy than might be expected. This lends support to the belief that the highest demands can (but not necessarily) occur on cold calm days (Grubb, 1988). The same conclusion can be drawn from Figure 1.11 from the data of hourly load factor and peak demand, derived from ten years of Great Britain electricity demand data and ten years of simulated wind-generation data, with each actual hour of wind speed matched against each actual hour of demand (OXERA, 2003). The graph shown in Figure 1.11 confirms that higher load factors (i.e. higher wind speeds) occur with higher percentage peak demands (i.e. winter demands). The significant section of the characteristic is in the droop shown in the load factors as peak demands approach the highest values. While not confirming that peak demands are always associated with the calm

Although attention is focused on variations in the overall national supply of wind power, it must not be forgotten that significant variations in supply could also come from more limited geographic areas if a large number of wind farms were contained therein. Significantly large concentrations of wind power generation capacity are being developed in Great Britain, both onshore and offshore; therefore, a two-year history of the output of a group of wind generators having rated capacities of 99MW in the Scottish Power Transmission area (southern Scotland) serves as a guide. These wind generators are geographically well distributed across the region; yet, between April 2003 and March 2005, for a total of 20.3 per cent of the half-hour periods, the aggregate output of all the wind farms was less than 5 per cent of the total capacity. Furthermore, for 12.6 per cent of the half hours, the output was less than 2 per cent of capacity, and in 2.2 per cent of the half hours there was no output at all (Bell et al, 2006). There is insufficient capacity in this example to have any significant impact on grid operations; nevertheless, the principle is clear that a region of the country with potential for considerable wind farm development can also experience large decreases in wind power output that should be evaluated in relation to the grid capacity requirements.

The relationship between hypothetical wind capacity and energy generated per annum for Great Britain can be seen in Figure 1.12. Again, a study of several years of hourly wind data gathered from Meteorological Office sites around the country, when processed through typical wind turbine power output versus wind speed characteristics, produced an annual probability

Source: National Grid (2002)

Figure 1.12 Probability distribution of total Great Britain wind power generation from 7600MW of dispersed wind turbines

distribution as shown (National Grid, 2002). Similar data distributions have been found for other countries.

Figure 1.12 shows the probability of achieving various power output levels from wind turbines over the whole country, given a theoretical total wind turbine-installed nameplate capacity of 7600MW. It is seen that the total average hourly power output calculated from Met Office wind data covering the country for every hour over the last five years can vary from 7300MW to practically zero. The mean output is 2270MW, which over a year would provide approximately 20 terawatt hours (TWh) of energy, or about 5 per cent of forecast national electrical energy demand in 2010, and would meet half of the government target for electrical energy generated from renewables.

Figure 1.12 is of fundamental importance in understanding the reasoning behind capacity credit estimates for various levels of wind penetration without resorting to mathematical explanations.